When marine oil leases bring bids as high as $140 million, it pays to know in advance what your chances are of finding oil in a particular block. Now a method known as "hydrocarbon micro-seep detection" or "sniffing" may make it possible for Gulf not only to predict whether a particular block has hydrocarbon potential, but also whether it is more likely to produce gas or oil. Since oil is quite a bit more valuable than gas, this information could have a direct impact on the amount Gulf should bid for a particular block.

The basic concept of sniffing involves analyzing samples of ocean water for minute quantities of light hydrocarbons which are associated with the presence of oil or gas. These minute quantities or "seeps" come from small fractures or faults in the ocean floor which tap deeper oil or gas reservoirs. By instant computer analysis, the presence, amount, and location of such seeps can be determined as a ship traverses several hundred miles of ocean. Additional processing can sometimes determine the type of source, that is, whether the reservoir is more likely to contain gas or oil.

When Gulf geophysicists are working from the Hollis Hedberg, the Company's geophysical ship, this information can be instantly related to faults or hydrocarbon traps beneath the ocean floor, which is being probed seismically at the same time. If a seep is detected in the same area as a favorable trap, the chances of locating a productive reservoir are increased.

Research Geochemist Jack Williams, in GS&T's Exploration and Production Division, explained why the two types of data can be used so well together:
"Seismic data is produced by sending sound waves deep into the earth below the ocean floor. The way these waves are reflected back from the rock layers indicates to the geophysicists where there are faults, traps, or formations that are likely to contain petroleum. Since many of these possible traps turn out to be void of petroleum, the presence of hydrocarbon seeps in the area being surveyed can add an additional confidence factor to the geophysicists' prospect. The presence of a seep is proof that the local hydrocarbon source beds have undergone all the necessary alterations to produce petroleum, the source being organic material which has matured or 'cooked' by temperature and pressure to a point where it has been converted into mobile hydrocarbons. Thus seismic information indicates the existence of trapping mechanisms, while geochemistry proves the existence of hydrocarbons. Neither condition independently, however, guarantees a commercial deposit."


Seep detection on the R/V Hollis Hedberg is accomplished by means of three tow-bodies which are towed behind the ship; one within the wave-mix zone or surface of the water; one at depths up to 400 feet; and one at 650 feet, or as close to the ocean floor as possible. Housed in each tow-body is a submersible pump and an instrument package for measuring temperature and tow depth. As water is pumped through a cable up to the ship, it enters a gas stripper. The hydrocarbons dissolved in the water are separated in a helium carrier which is passed on into a gas chromatograph, to be analyzed for light hydrocarbons. Testing is usually done for methane, ethane, propane, and iso- and normal butane, the hydrocarbons which are petroleum-related. The gas analysis is also logged onto magnetic tape for additional processing later at the Harmarville lab.
The reason for testing at three different depths, explained Mr. Williams, is that a seep will "plume out" and become broader as it rises to the surface. This increases the area in which it can be detected by the surface tow-body. The tow-body close to the ocean floor, on the other hand, can pinpoint the source of the seep more closely. The mid-depth tow-body acts as a control for background traces of hydrocarbons which are always present in the water.

Another advantage of having three inlets is that this method tends to rule out hydrocarbons coming from oil spills or other surface contamination. Oil spills floating in the wave-mix zone would show up only from the surface tow-body and so would be disregarded in exploring for seeps.

By using geochemical techniques, geochemists have recently discovered that several known commercial reservoirs of natural gas are actually biogenically produced. This means that instead of being formed by the thermal processes of temperature and pressure, such gas is produced by large colonies of bacteria acting on organic material buried under layers of sediment. The significance of this discovery, said Mr. Williams, is that in the past, biogenic methane seeps have largely been ignored because they were considered to indicate shallow gas reservoirs too small to be commercial. This recent development, however, has caused Gulf to look more carefully at dry methane seeps and to consider their possible exploration significance.

Recently, Gulf has employed the marine sniffer technology in the Gulf of Mexico and several foreign countries such as Venezuela, Indonesia, and the Cameroons.